Current comparator device having plural magnetic cores and multiple windings



1964 N L. KUSTERS ETAL 3, ,7

CURRENT COMPARATOR DEVICE HAVING PLURAL MAGNETIC CORES AND MULTIPLEWINDINGS Filed Dec. 26, 1961 S Sheets-Sheet 1 PRIMARY wmomq /9 L /8 fNfi N SECONDARY wmomq w? p EXTERNAL lsb EXCITATlON WINDING 3,, COREWINDNQ m ELECTROSTATKZ swam DETEC-TKDN wmomq Arron/27.:

Oct. 20, 1964 N. L. KusTERs ETAL 3,153,758

CURRENT COMPARATOR DEVICE HAVING PLURAL MAGNETIC CORES AND MULTIPLEWINDINGS Filed Dec. 26, 1961 3 Sheets-Sheet 2 VOLTAGE. POWER Bounce.$uPPLY PRMARY cmcun l f QMPARATQR TRLhLBEORMER V'RIMARY wmbmc, /9 m T /8v SECONDARY wmomc. W

D2 EXUTATION wmoma Z BURDEN Davmnou wmomqoewacnou wmomr.

r r SecounARY -Z:II-E CHZCLHT PRMARY CIRCLHT 7 COMPARATOR TRAN SFO RM ERPRM ARY W'NDING SECONDARY ClRCUtT SEcouDARY wmomc,

Excn a'moN auxoeu wmomc DEVIATION WIHDUJG DETECTION wluomq VOLTAGE.PawER SoulcE. SUPPLY 1964 N. L. KUSTERS ETAL 3, ,7

CURRENT COMPARATOR DEVICE HAVING PLURAL MAGNETIC CORES AND MULTIPLEWINDINGS Filed Dec. 26, 1961 3 Sheets-5heet 3 SECONDARY cmcun' PHANTQMBURDEN United States Patent M CURRENT COMPARATOR DEVICE HAVING PLURALMAGNETIC CORES AND MUL- TIPLE WINDINGS Norbert L. Kusters, Petal N.Miljanic, and William J. M. Moore, Ottawa, Ontario, Canada, assignors toNational Research Council, Ottawa, Ontario, Canada, a body corporateFiled Dec. 26, 1961, Ser. No. 162,033 13 Claims. (Cl. 324-55) Thisinvention relates to an improved device for comparing alternatingelectric currents, said, in particular, is concerned with improvementsin a current comparator of the type which consists of a pair of windingsmounted on a toroidal or other magnetic core closed on itself to torn anendless magnetic circuit, said windings being so connected as togenerate magnetizing force opposing each other. When the currents areequal, assuming the winds ings have an equal number of turns (or moreaccurately, when the ampere-turns are equal, since the number of turnsneed not necessarily be equal), there will be zero flux in the core.Athird, detection winding is connected to a suitable null indicator toshow when balance has been achieved.

In some current comparators, a fourth winding known as the deviationwinding is provided, thi winding having injected into it from a suitablycontrolled source a current required to effect perfect balance Forexample, whenever there is a phase difference between the currents inthe two main windings, some quadrature current will be required forbalance, and this can be supplied through the deviation winding.

The present invention contemplates a current comparator that isfurnished with the two main windings (which for convenience will bereferred to henceforth as the primary and the secondary windings); thedetection winding; a deviation winding, a an optional but normallydesirable feature; and an additional winding known as the excitationwinding. The excitation winding is divided into two parts, such partsbeing positioned on opposite sides (radi- Patented Oct. 20, 1964 In thedrawings:

FIGURE 1 shows a diagrammatic cross-section of a current comparatordevice according to the invention;

FIGURE 2 is a first circuit providing an example of a use for suchdevice;

FIGURE 3 is a fragment of FIGURE 2 showing a modification; and

FIGURE 4 is a second circuit providing a further example of a use forthe device.

The comparator device shown in FIGURE 1 is toroidal in form and each ofthe windings shown is assumed to ally) of a second magnetic core thatextends parallel to p and preferably coaxially around the principalmagnetic core and is also closed on itself to form an endless magneticcircuit. The two parts of the excitation winding which are respectivelyinternal and external to the second core are so connected (for example,series opposition) as to generate equal and opposite fluxes, and henceno net flux, within the main, inner core which they both surround. Theythus have no effect on the detection winding. However, the second coreis mounted within only the external partof the excitation winding andconsequently it has a flux set up in it, which flux cuts the primary andsecondary windings.

As will appear more fully from the description thatfollows this effectcan be employed either to generate a desired voltage in the primary orsecondary winding, or to cause power transfer in transformer fashionbetween the primary and secondary windings, such effect being achievedwithout interfering with the proper comparator operation of thesewindings in relation to the detection and deviation windings. I

In further illustration of these features of the invention, theaccompanying drawing show one constructional example of a comparatordevice, and two circuits in which advantage can be taken of theoperation of such device. It is to be understood that theseillustrations are provided by way of example only, that the comparatordevice may take other forms and that its operational features may bemade use of in other applications and circuits, the scope of theinvention being limited only by the appended claims.

extend therearound. At the axial centre of the device is the primarymagnetic core 10. In place around the core 18* is a detection winding11, which will thus indicate flux in the core 10. Radially outwardlyagain there may conveniently be provided a copper electrostatic shield12 split along edge 13 in the usual Way to avoid a shorted turn. Thisshield 12 protects the detection winding 11 from interference from strayelectric fields.

A deviation winding 14 is in next position. This winding has beenillustrated, since it is provided in the preferred form of the inventionand is normally necessary, if full balance is to be achieved. It is not,however, fundamental to the inventive advance in its broadest form.

Over the deviation winding, there is placed a first part 15a (known asthe internal part) of an excitation Winding. Then, radially outwardly ofwinding 15a, there is mounted in the assembly a second magneticlaminated core 16. This core 16, which also performs the function ofmagnetic shield for the detection winding within it, has been shown asmade up of four sets of laminations, arranged one set against each ofthe four faces of the assembly. To prevent a shorted turn, at least onespacer 17 is employed. Like core 10, core 16 is closed on itself tocomplete an endless magnetic circuit.

Immediately outside the core 16, there is placed the second, externalpart 15b of the excitation winding. The winding parts 15a and 15b havean equal number of turns and are connected in series opposition.

Radially outwardly of the winding part 15b, are positioned the two mainwindings, secondary 18 and primary 19. In practice, more than two suchmain windings may be needed, in which case they will all be placed inthis outer position. Windings 18 and 19 and winding part 1515 thusconstitute outer windings extending around both cores 10 and 16.

In practice, additional shielding may be provided, as found desirable.Such features have been omitted, from the drawing, since the presentdescription is concerned with the fundamental principle of operationrather than with practical constructional considerations.

Since the two parts 15a and 15b of the excitation winding are connectedin series opposition, their fiux generating effects on the primary core10 (which is situated within both such winding parts) cancel one anotherout. Hence the excitation winding produces no flux in core 10. But thesecond core 16 is unaifected by current in winding part 154': whiclritis positioned radially beyond, and consequently'flux is generated incore 16 by the unopposed external part 15b of the excitation winding.This flux has no effect onthe detection winding 11 which lies whollyradially inwardly of core 16, but it does induce voltages in the primaryand secondary windings 19 and 18 which surround the core 16. V

. The device, as a whole, can thus be considered as being It is first acom-' parator, as between the oppositely connected primary two devicesat one and the same time.

excited by the external part of the excitationwinding, the

6. detection Winding being entirely insensitive to this function.

A particularly convenient use of such a dual function device isillustrated in FIGURE 2, which shows the device employed for thecalibration of a current transformer. Current transformer T (assumed forsimplicity to have a nominal 1 to 1 ratio) is under test, and itsprimary is connected in series with primary Winding 19 of the comparatordevice. Current is supplied from power supply P. Secondary Winding 18 ofthe comparator device is series connected with the secondary oftransformer T and a standard burden Z. With equal and opposite currentsin windings 18 and 19, the null detector DI connected to detectionwinding 11 should show no reading. In practice it will show a smallreading, because of the error in trans former T that is to becalibrated. A correcting flux has to be supplied to the core by means ofan error current injected into the device by way of deviation winding14. This error current has an in-phase component adjusted by variableresistor R and a quadrature component adjusted by variable capacitor C.These components of the error current are made to be related inmagnitude and phase to the current in the secondary circuit by beingderived from voltages across resistors r in the secondary circuit. Therequired sign is determined by switches X and Y.

If the parts of which the function has so far been described were theonly components present, a calibration could be obtained, butdisadvantages would accrue in accuracy or convenience of measurement byreason of the effect of the impedance of the components added to theburden Z (winding 18 and resistors r) in the transformer secondarycircuit.

This disadvantage can be overcome by use of the excitation winding (15a,1515). This excitation winding is energised from an adjustable voltagesource VS, adjustable, that is, in magnitude and phase in relation tothe primary current supplied by power supply P and hence the current inthe secondary circuit. In addition, a further null detector D2 isconnected between the point common to the transformer and comparatorsecondaries and the point common to the burden Z and resistors r. Whenthe excitation current has been adjusted to give no reading on detectorD2 it means that there is now no net voltage drop in the secondarycircuit across winding 18 and resistors r, and these componentsconsequently represent no impedance in the secondary circuit. Theseconditions are thus equivalent to the secondary of transformer T beingconnected directly across the burden Z, the ideal condition.

The manner in which balance of detector D2 is achieved is to so adjustthe supply to the excitation winding that it induces in the secondarywinding 18 of the com parator a voltage equal and opposite to thevoltage that would otherwise appear across detector D2 from the voltagedrop in winding 18 and resistors r due to the current in the secondarycircuit. Once this balance has been achieved, the detector D1 is alsobrought to a null by adjustment of the values of resistor R andcapacitor C, which components can be made to give a direct reading ofthe in-phase and quadrature errors of the transformer T undercalibration.

A further advantage of this method of operation lies in its permittingthe use of a phantom burden in lieu of an actual burden. A phantomburden is simpler and cheaper, and dissipates far less power than a realburden. It also provides a four quadrant burden (including a negativeresistance) which cannot otherwise be realised. FIGURE 3 illustrates bymeans of a fragment of FIG- URE 2 such a phantom burden arrangement, theremainder of the circuit being assumed to be the same as that of FIGURE2. The phantom burden consists of a resistor R1 across which isconnected a portion of the coil (n2 turns) of an auto-transformer T1,the polarity of such connection being reversible by double pole, doublethrow switch S1. A mutual inductor M1 has a first windtmg (n3 turns) inseries with resistor R1 and a. second winding connected at one end to atap on transformer T1. Detector D2 is connected between a tap on thesecond winding of inductor M1 and the point common to winding 18 and thesecondary of transformer T. The polarity of the latter connections isalso reversible by a second double pole, double throw switch 82.

This circuit is used in the same manner as before, excitation of theexcitation winding being adjusted to bring detector D2 to a null. Underthese conditions the voltage across the secondary of transformer T isthe same as it would be if said secondary were connected across theactual impedance which the phantom burden is simulating. This simulatedburden consists of a resistive component equal to :RlXturns ratio ontransformer T1 (ml/n2) where at is the number of turns tapped off by thetap on transformer T1, and a series connected inductive component equalto ijwM, where M is variable and is the mutual inductance between turnsn3 and n4, where 114 is the number of turns tapped off by the tap on thesecond winding of inductor M1.

A modification to the circuit of FIGURE 2 is illustrated in FIGURE 4 anddemonstrates the transformer-like function of the outer windings of thecomparator. In this circuit, the primary circuit has been shortcircuited on itself; and power has been supplied to the secondarycircuit. The excitation winding, as before, is energised from a voltagesource VS adjustable in magnitude and phase in relation to the secondarycurrent.

An ammeter A in the secondary circuit is used first to determine when asuitable chosen current value has been achieved, by adjustment of thecurrent source. The excitation winding supply is then adjusted. Underthese conditions the primary and secondary windings of the comparatordevice act as a power transformer, feeding from the secondary winding 18to the primary winding 19 to set up a current in the closed primaryloop. The power transfer that can be effected in this way is large incomparison with the power that must be supplied to the excitationwinding to make the transfer possible. This circuit is also capable ofoperating with a phantom burden in a like manner to FIGURE 2 as modifiedby FIGURE 3.

When the correct excitation conditions prevail, detector D2 will show anull, and then detector D1 can be brought to a null by adjustment ofresistor R and capacitor C as before, the windings l8 and 19simultaneously acting as opposed comparator windings in conjunction withcore 10 at the same time as they act as transformer windings with core16.

An important advantage of the application of the present invention tocurrent transformer testing is the ability which it furnishes tocalibrate a transformer in situ, since no heavy equipment is required.

We claim:

1. A current comparator device comprising (a) an inner magnetic coreclosed on itself to form a magnetic circuit and define a longitudinalaxis extending along and around said core,

(b) a detection winding around said core,

(c) an internal part of an excitation winding around said core,

(d) a second magnetic core closed on itself to form a magnetic circuit,said second core extending coaxially with said longitudinal axis of theinner core and being positioned radially outwardly of both saiddetection winding and said internal part of the excitation Winding,

(e) an external part of the excitation winding extending around bothsaid cores and having the same number of turns as said internal part,

(f) and primary and secondary windings extending around both said cores.

2. A device according to claim 1, including a deviation windingextending around said inner core.

3. A device according to claim 1, including an electrostatic shieldextending immediately around said detection winding.

4. The combination of (a) a primary circuit and a secondary circuit, thecurrents in which are to be compared,

(b) a current comparator device comprising,

(i) an inner magnetic core closed on itself to form a magnetic circuitand define a longitudinal axis extending along and around said core,

(ii) a detection winding around said core,

(iii) a deviation winding around said core,

(iv) an internal part of an excitation winding around said core,

(v) a second magnetic core closed on itself to form a magnetic circuit,said second core extending coaxially with said longitudinal axis of theinner core and being positioned radially outwardly of both saiddetection winding and said internal part of the excitation winding,

(vi) an external part of the excitation winding extending around bothsaid cores and having the same number of turns as said internal part,said parts being connected together in series opposition,

(viii) and a pair of primary and secondary windings extending aroundboth said cores, said primary and secondary windings being connectedrespectively in said primary and secondary circuits,

(c) means connected to said detection winding for detecting zero outputtherefrom,

(d) means connected to said deviation winding for passing a currentthrough said deviation winding, said means including means forcontrolling the magnitude and phase of such last-mentioned current inrelation to the current in said secondary circuit whereby to bring theoutput of said detection winding to zero,

(e) and means for exciting said excitation winding parts whereby togenerate no flux in said inner core and no output in said detectionwinding while generating flux in said second core and inducing a voltagein said secondary Winding, said last-mentioned means including means forcontrolling the magnitude and phase of the current in said excitationwinding in relation to the current in the secondary circuit for bringingthe voltage induced in said secondary winding to a predetermined value.

5. The combination of claim 4, including (a) a current transformer,

(b) means series connecting the primary and secondary windings of saidcurrent transformer respectively in said primary and secondary circuits,

() and a power supply connected to said primary circuit for transfer ofcurrent to said secondary circuit through said current transformer.

6. The combination of claim 4, including (a) a current transformer,

(b) means series connecting the primary and secondary windings of saidcurrent transformer respectively in said primary and secondary circuits,

(0) and a power supply connected to said secondary circuit for transferof current to said primary circuit from the secondary winding of thecurrent comparator device to the primay winding of the currentcomparator device.

7. The combination of claim 4, including (a) a current transformer,

(b) means connecting the primary winding of said current transformer insaid primary circuit in series with the primary winding of the currentcomparator device,

(c) a burden connected in said secondary circuit in series with thesecondary winding of the current comparator device and the secondarywinding of the controlling the magnitude and phase of the current passedthrough the excitation winding includes an impedance connected in seriesin said secondary circuit.

9. The combination of claim 4, including (a) a current transformer,

(12) means series connecting the primary and secondary windings of saidcurrent transformer respectively in said primary and secondary circuits,

(0) a burden connected in said secondary circuit directly to and inseries with the secondary winding of the current transformer,

(d) and means for detecting a voltage connected across the seriesconnection of burden and current transformer secondary winding.

10. The combination of claim 9, wherein said burden comprises (a) animpedance connected in said secondary circuit,

([2) transformer means connected to said impedance for generating avoltage proportional to the voltage drop across said impedance.

11. The combination of claim 10, wherein said impedance comprises aresistive portion for generating an in-phase voltage proportional tocurrent in said secondary circuit, and an inductive portion forgenerating a quadrature voltage proportional to said current.

12. A current comparator device comprising (a) an inner magnetic coreclosed on itself to form a magnetic circuit and define a longitudinalaxis extending along and around said core,

(b) a detection winding around said core,

(c) a second magnetic core closed on itself to form a magnetic circuit,said second core extending coaxially with said longitudinal axis of theinner core and being positioned radially outwardly of said detectionwinding,

(11) at least two outer windings extending around both said cores,

(e) and a further winding positioned radially between said cores,

(1) said further winding having the same number of turns as a selectedone of said outer windings.

13. The combination of a current comparator device according to claim 12with (a) a primary circuit and a secondary circuit, the currents inwhich are to be compared,

(12) means connected to said detection winding for de tecting Zerooutput therefrom,

(c) means connecting two of said outer windings respectively in saidprimary and secondary circuits,

(d) means connecting said further winding in series opposition with saidselected outer winding,

(2) means for exciting said serially connected windings whereby togenerate no flux in said inner core while generating flux in said secondcore and inducing a voltage in each of the outer windings,

(f) means for producing an error current including means for controllingthe magnitude and phase of such error current in relation to the currentin said secondary circuit,

(g) and means for injecting said error current into said device.

References Cited in the file of this patent UNITED STATES PATENTS316,817 Perrin Apr. 28, 1885

1. A CURRENT COMPARATOR DEVICE COMPRISING (A) AN INNER MAGNETIC CORECLOSED ON ITSELF TO FORM A MAGNETIC CIRCUIT AND DEFINE A LONGITUDINALAXIS EXTENDING ALONG AND AROUND SAID CORE, (B) A DETECTION WINDINGAROUND SAID CORE, (C) AN INTERNAL PART OF AN EXCITATION WINDING AROUNDSAID CORE, (D) A SECOND MAGNETIC CORE CLOSED ON ITSELF TO FORM AMAGNETIC CIRCUIT, SAID SECOND CORE EXTENDING COAXIALLY WITH SAIDLONGITUDINAL AXIS OF THE INNER CORE AND BEING POSITIONED RADIALLYOUTWARDLY OF BOTH SAID DETEC-